A customer brought me a Penn 965, on which the mechanism locks (handle only turns ,with no retrieval, under great force) when a fish is on line. Reel is used for strippers in FRESH WATER only. The reel will free up when the freespool button is pushed, but locks up again very quickly. I disassembled the right side of the reel checked the gears and freespool mechanism, all of which seem fine. The worm and line guide seem to work normally. I have not disassembled the left side at this point. Does anyone experienced with these reels have any suggestions on how to diagnose and fix this issue? Your ideas are greatly appprecited.

Yes. I believe your problem is in the left side, which you need to disassemble to troubleshoot. On the older 965's the idler gear was plastic with a brass core which would tend to come free creating slippage, and then causing the teeth to mesh improperly with those of the level wind worm gear. A mess. You may find shredded pieces of white plastic inside the sideplate, that's the giveaway. Anyway to do a complete fix you need to get the fresh idler gear, level wind worm gear and the white plastic gear that fits the end of the left side of the spool - the one with the four thin metal spikes in it that hold the centrifugal brake blocks. I've seen the metal spikes come free from the plastic gear assembly, bollixing up the works as well.

Thank you, I replaced the main gear ratchet and a bent clutch arm. I tested the reel under much tension, and it seemed to work. Now the customer, a Striper guide, has brought it back saying it does not work. I tested it, again, with up to 8 lbs tension and it worked fine. It then locked up with about 11 pounds on the line. The customer says that he fishes with six pounds of weight on it (?). It will not crank consistently with a fish from 10 up to 30 pounds.

I really appreciate the suggestions you have provided. I will check the left side and level wind once more.